The present invention relates to ion chamber radiation detectors in which an electrical current is generated in response to the radiation field of the surrounding environment. Such ion chambers rely upon the interaction of the incident radiation with the fill gas within the chamber to produce an electrical current through the gas between voltage biased electrodes. The electrical current produced is a function of the radiation impinging on the chamber. Ion chambers are widely used as radiation monitoring devices, for example in nuclear reactor containment environments.
It is desirable for detectors of this sort to have a response in amperes per roentgen per hour which is independent of the energy of the gamma-rays impinging on the chamber. If this is true then the chamber correctly indicates the health hazard associated with a gamma field regardless of the energy spectrum of the field. This is most easily achieved by use of air or tissue equivalent radiation monitoring chambers, which are constructed of low atomic weight organic materials to simulate either air or tissue.
In ion chambers designed for post-accident nuclear reactor environments, the ion chamber must be capable of withstanding intense radiation, high pressure, high temperature, and even corrosive chemical reactants. These conditions eliminate the use of organic materials for construction of the detector, and dictate that high temperature resistant metallic members be used. However, metal walled ion chambers do not exhibit the same energy independent response attained with air and tissue equivalent detectors. It has been found possible in prior art metal walled ion chamber designs to satisfy the energy independent requirement when the fill gas was maintained at approximately atmospheric pressure. Such low fill pressure ion chamber designs however suffer from reduced sensitivity. The most widely utilized fill gas in such ion chambers is nitrogen. The more sensitive, high fill pressure, metal walled ion chambers of the prior art typically utilize nitrogen gas at a fill pressure of up to about 10 atmospheres. Such high pressure, high sensitivity, walled ion chambers do not exhibit the requisite flat energy response characteristic. Recent regulations for such accident monitoring ion chambers call for an energy response which is flat within .+-.20% of the mean value. The high-pressure nitrogen fill gas ion chamber fails to meet this criteria because the response decreases significantly at low gamma ray energies.
It has generally been recognized that for many ion chamber designs having low atomic number gas fills, such as nitrogen, a decreased response characteristic is observed at low energies of the gamma radiation field. This is particularly true where metal walled thick electrode structures are utilized. It is also known that for ion chambers with high atomic number fill gases such as xenon, the signal response increases significantly at low gamma ray energies.